Copper phthalocyanine as synthesized is referred to as crude copper phthalocyanine and is not suitable as a pigment for printing ink since it consists of large .beta.-form crystal particles having a particle size of approximately 10 to 200 .mu. m. The process to reduce the size of this crude copper phthalocyanine to a size (approximately from 0.02 to 0.1 .mu.m) suitable for printing ink is called pigmentation. Printing ink is obtained using pigmentized copper phthalocyanine.
There are various methods for pigmentation. The most common method is salt milling which uses a solvent (hereinafter referred to as a solvent salt milling method). This method is one in which a milling agent such as sodium chloride and an organic solvent which promotes crystal transformation to the .beta.-form are added to crude copper phthalocyanine and the resulting mixture is subjected to the milling process. The .beta.-form copper phthalocyanine pigment produced by this method is suitable for printing ink and is widely used since the aspect ratio of the pigment particle is from 1 to 2 and it has clear greenish hue with strong color strength. However, this method requires an amount of milling agent several times that of the pigment and also consumes considerable time, labor and energy in recovering the milling agent and organic solvent.
An alternative method is known in which crude copper phthalocyanine is dry milled and then treated with an organic solvent. As a portion of the .beta.-form crystals are transformed to .alpha.-form crystals in this process due to the mechanical force applied during milling, the ground material is heat treated with an organic solvent in order to transform the .alpha.-form crystals back to .beta.-form crystals. This method is favourable costwise as compared to the solvent salt milling method but involves problems such as the increase in the aspect ratio of the pigment particle due to acicular growth of the particles during heat treatment in organic solvents, the hue turning reddish and the decrease in fluidity etc. Methods are known for suppressing particle growth by adding pigment derivatives or growth inhibitors but these additives are usually unfavourable ingredients for printing ink.
On the other hand, for production of printing ink from pigments, methods using dry pigments or wet cake pigments which contain 40-70 weight % of water are generally used. The method using wet cake pigments is called flushing method. In the method using dry pigments, the pigment is dispersed using beads mill, three roll mill, etc. after mixing the dry pigment with printing ink varnish, solvents, additives, etc. A large amount of energy is required, however, to disperse the pigment as the primary particles of dry pigments are susceptible to aggregation. In the flushing method, the pigment is converted from the water phase to the varnish phase by mixing the wet cake with printing ink varnish, solvent, additives, etc. Although the energy required for this method is less than that of the dry pigment method, large installations such as a kneader are necessary and drainage occurs during the flushing process.
As indicated above, the process of pigmentation and the production of ink requires a large amount of time and energy for producing printing ink from .beta.-form copper phthalocyanine.
In order to provide low cost ink, production of ink directly from crude copper phthalocyanine without going through the stage of pigmentation may be conceived. However, the milling efficiency is low when printing ink is produced directly by mixing crude copper phthalocyanine with printing ink varnish, because the conventional pigmentation process is performed in the presence of printing varnish in this case. For instance, the use of a dispersing mill using ultrafine beads require a large amount of energy. And moreover, the quality of the product is problematic.
In Japanese Patent Publication No. S55-6670, a method for producing ink from crude copper phthalocyanine dry ground once is described. In this method, dry grinding is effective as it can be performed efficiently. But, due to the fact that the resulting ground material is a mixture of .alpha./ .beta.-form crystals, the .alpha.-form crystals in the ink has to be reconverted to .beta.-form crystals. Transformation from the .alpha.-form crystals to .beta.-form crystals may be effected by heating in an organic solvent. However, although this reconversion to .beta.-form proceeds smoothly in gravure ink, progress is hampered in offset lithography ink. And the transformation to .beta.-form crystals is very difficult in solvents such as AF Solvent (trade name), for which demand has enlarged in recent years. Furthermore, means to suppress crystal growth which proceeds in parallel with the crystal transformation is necessary.
It is known that in order to produce ink from ground copper phthalocyanine material efficiently, mitigation of the aggregation of the ground copper phthalocyanine is very effective and several methods have been proposed.
UK Patent No. 1224627 describes a method in which resin 1-8 times in quantity of the crude copper phthalocyanine is added when dry grinding crude copper phthalocyanine. The aggregation of copper phthalocyanine particles is mitigated by this method. However, the addition of such a large amount of resin incurs increased danger in resin sticking inside during dry milling. The temperature during milling must be held low to prevent sticking. However, it is difficult to obtain ground material with a high content of .beta.-form copper phthalocyanine when milled at low temperatures. Moreover, due to the large aspect ratio of the pigment particles in the ink, obtained from the pigments ground according to this method, the ink assumes a reddish hue and cannot be said to be of desirable quality.
Japanese Patent Application Laid-Open No. H2-294365 discloses a method wherein resins such as rosin modified phenolics are added to crude copper phthalocyanine by 0.5-10 weight % based on the crude copper phthalocyanine in the process of dry grinding. This method necessitates a large amount of energy in the process of dispersion into printing ink varnish. Therefore, this method cannot be said to be superior to conventional printing ink production methods which use dry pigments. Moreover, methods are not presented to keep the aspect ratio of the pigment particles small in the printing ink and this problem remains unsolved.